HD 7750, hands down.
Make sure you get the version that doesn't require pins though, there are some versions that require a 6-pin connector - but these ones are designed for overclockers.
I'd recommend the XFX HD 7750 Core Edition (1gb GDDR5). It is a superb card.
Yes, there is a single-slot version 🙂
Get the XFX HD 7750 single-slot version.
XFX cards have good cooling, and that's necessary for single-slot cards, as they get hot quicker.